1. Field of the Invention
The invention relates to a process for producing a planar body of an oxide single crystal.
2. Description of the Related Art
A single crystal of lithium potassium niobate and a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution have been noted especially as single crystals for a blue light second harmonic generation (SHG) device for a semiconductor laser. The device can emit even ultraviolet light having wavelengths on the order of 390 nm, thus the crystals can be suitable for wide applications such as optical disk memories, medical and photochemical fields, and various optical measurements by using such short-wavelength lights. Since the above single crystals have a large electro-optic effect, they also can be applied to optical memory devices using their photo-refractive effect.
However, for an application of a second harmonic generation device, for example, even a small fluctuation in the composition of the single crystal may affect the wavelength of the second harmonic wave generated by the device. Therefore, the specification of the range of the composition required for such single crystals is severe, and any fluctuation in the composition should be suppressed to a narrow range. However, since the composition consists of as many as three or four components, growing a single crystal at a high rate is generally extremely difficult to achieve, while controlling the proportion of the components to be constant.
In addition, for optical applications, especially for an application for the second harmonic wave generation, a laser beam having a short wavelength of, for example, about 400 nm needs to propagate in the single crystal at the highest power density possible. Moreover, photo deterioration has to be controlled to the minimum at the same time. In this way, since controlling the photo deterioration is essential, the single crystal has to possess a good crystallinity for this purpose.
Moreover, lithium niobate and lithium potassium niobate can be substituted with cations, thus a solid solution, in which the cations are solid-solved, is produced. Therefore, controlling the composition of the melt is necessary to grow a single crystal of a specific composition. From such a background, a double crucible method and a method of growing a crystal while feeding raw materials are examined mainly for the CZ method and the TSSG method. For example, Kitamura et al. tried to grow a lithium niobate single crystal of a stoichiometric composition by combining an automatic powder feeder to a double crucible CZ method (J. Crystal Growth, 116 (1992), p.327). However, it was difficult to enhance the crystal growth rate with these methods.
NGK Insulators, Ltd. suggested a micro pulling-down method for growing the above single crystal with constant compositional proportions, for example, in JP-A-8-319191. In this method, a raw material, for example, lithium potassium niobate, is put into a platinum crucible and melted, and then the melt is pulled down gradually and continuously through a nozzle attached to the bottom of the crucible. The micro pulling-down method can grow a single crystal more rapidly than the CZ method or the TSSG method. Moreover, the compositions of the melt and the grown single crystal can be controlled by growing the single crystal continuously with feeding the raw materials for growing the single crystal to the raw material melting crucible.
However, there is still a limitation in using a micro pulling-down method to grow a good single crystal plate (a planar body of a single crystal) continuously at a high rate. That is, when the planar body of the single crystal is pulled down with a planar seed crystal, cracks tend to occur near an interface boundary between the seed crystal and the planar body.
The inventors found that cracks were likely to occur as the difference in lattice constant between the seed crystal and the planar body became greater, and suggested a method for preventing cracks by matching the lattice constant of the seed crystal with that of the planar body at a high accuracy in Japanese Patent application No. 2000-065123 (filed on Mar. 9, 2000). However, it was practically very difficult for a multi-component solid solution system as in lithium potassium niobate-lithium potassium tantalate solid solution to match the lattice constant of a seed crystal with that of a planar body at a high accuracy.
It is an object of the invention to prevent cracks from occurring near the interface boundary between a seed crystal and a planar body, and to grow a planar body of an oxide single crystal having a good crystallinity continuously and stably with a simple technique, when the planar body of the oxide single crystal is grown with the micro pulling-down method.
The inventors examined various methods to grow planar bodies of oxide single crystals which used the micro pulling-down method. As a result, the inventors found that cracks were more unlikely to occur as the area of the interface boundary between the seed crystal and the planar body became smaller. Thus, a planar body having a good crystallinity was continuously made by melting a raw material of an oxide single crystal in a crucible, contacting a fibrous seed crystal to a melt, pulling down the melt from the opening of a crucible by lowering the seed crystal, forming a shoulder portion following the seed crystal, forming a planar body following the shoulder portion, and controlling differences in lattice constants between each crystal axis of the seed crystal and each corresponding crystal axis of the shoulder portion at 1% or less (more preferably 0.5% or less), respectively.
In this case, the lattice constant of each crystal axis of the shoulder portion can be adjusted by controlling the proportions of respective components in the crucible. Taking lithium potassium niobate for an example, the lattice constants of each crystal axis in the grown planar body can be changed by slightly changing the relative ratio of niobium, lithium and potassium in the crucible.